The present invention relates to a semiconductor device and, more particularly, to a molecular controlled semiconductor sensing device.
Semiconducting materials are well known and include n-type and p-type semiconductors, so named because either they have an excess of electrons (“−”; n-type semiconductor) or a deficit of electrons than what is necessary to complete a lattice structure (“+”; p-type semiconductor). The extra electrons in the n-type material and the holes (deficit of electrons) left in the p-type material serve as negative and positive charge carriers, respectively. Semiconductor devices typically include at least one p-n junction, which is a border region between an n-type and p-type semiconductor. The p-n junction possesses properties, which can be used in many electronic applications, such as diodes, transistors, memory media and the like.
A diode is an electronic device that allows current flow (i.e., electronic conduction) in one direction but prevents current flow (i.e., is insulating) in the opposite direction. Commonly, the conductive and insulating states of a diode are referred to as a “forward bias” and “reverse bias” effects, respectively, where the term “bias” corresponds to the application of electric voltage to the p-n junction.
In forward bias, the holes in the p-type region and the electrons in the n-type region are pushed towards the p-n junction. Application of a forward bias is by connecting the p-type region to a positive terminal of a voltage source and the n-type part to a negative terminal of the voltage source. With such voltage configuration, the positive charge applied to the p-type region repels the holes, while the negative charge, applied to the n-type region, repels the electrons. This reduces the junction barrier, allowing the electrons to overcome this barrier and enter the p-type region. Once inside the p-type region, the electrons make their way to the positive terminal of the power supply, hence generating an electric current.
In reverse bias, the p-type region is connected to the negative terminal and the n-type region is connected to the positive terminal of the power supply, thus pulling the holes in the p-type and the electrons in the n-type away from the p-n junction. This effectively widens the p-n junction, increasing the electrical resistance to the flow of electrons. Up to a certain voltage, commonly referred to as the breakdown voltage, regular diodes practically prevent current flow therethrough. By exceeding the breakdown voltage, a regular diode is destroyed due to excess current and overheating.
A Zener diode is a diode device especially designed to have a greatly reduced breakdown voltage, also known as the Zener voltage, named after C. M. Zener (1905-1993). A Zener diode contains a heavily doped p-n junction allowing electrons to tunnel from the valence band of the p-type material to the conduction band of the n-type material. A current-voltage curve characterizing the Zener diode comprises a region of forward current at a forward voltage (about 0.7 volts for silicon diode), a region of reverse or breakdown current at the Zener voltage and a flat region of small (practically zero) current therebetween. Thus, upon application of a reverse bias, the Zener diode exhibits a controlled breakdown and lets current flow in a manner such that the voltage across the Zener diode is kept at the Zener voltage. The breakdown voltage of a Zener diode can be accurately controlled in the doping process of the semiconductor materials forming the p-n junction.
An avalanche diode is another diode device designed to provide a breakdown current. It this device, the breakdown is by impact ionization rather than by the Zener effect. When no or small reverse voltage is applied to the avalanche diode, thermal energy results in formations of a few electron-hole pairs in the depletion region of the p-n junction. When a sufficient reverse voltage (i.e., above the breakdown voltage) is applied across the p-n junction, the electrons accelerate in the electric field, collide with the atoms of the semiconductor lattice, and rupture their covalent bonds to form more pairs. The released electrons also accelerate in the electric field, resulting in a chain or avalanche effect of carrier multiplication in which further electron-hole pairs are released. The avalanche effect releases an almost unlimited number of carriers so that the avalanche diode essentially becomes a short circuit. The current flow in this region is limited only by an external series current-limiting resistor. Once the reverse voltage is removed, all the charge carriers return to their normal energy values and momenta.
A Schottky diode is a diode device which, unlike the p-n junction of a conventional diode, has a metal-semiconductor junction in which the work function of the metal and the band gap of the semiconductor are selected to reduce the voltage across the junction. Such junction is termed Schottky barrier, after Walter H. Schottky (1886-1976).
A field effect transistor (FET) is a semiconductor device having a source electrode, a drain electrode, a gate electrode and a channel, which is separated from the gate electrode by a thin isolating layer. The channel has semiconducting properties (either n-type or p-type semiconducting properties) such that the density of charge carriers therein can be varied. When no voltage is applied to the gate electrode, the channel does not contain any free charge carriers and is essentially an insulator. Upon application of a certain level of voltage to the gate electrode, an electric field, generated between the channel and the gate, attracts charge carriers (electrons or holes) from the source electrode and the drain electrode, and the channel becomes conductive.
Semiconductor materials and devices can be used as transducers in sensing applications whereby semiconductor materials are combined with a sensing element responsive to chemicals or energy. For example, in U.S. Pat. No. 4,777,019, the contents of which are hereby incorporated by reference, small monomers of macromolecules are directly introduced into the surface layer of a semiconductor, to thereby form a biosensor having an improved signal to noise ratio. The use of semiconductor materials as transducers in sensing devices is an attractive option because sensing devices employing other transducers (e.g., electrochemical, piezoelectric or optical transducers) suffer from various limitations, including high cost, complexity and/or bulkiness. Contrarily, the combination of semiconductors with sensing molecules enjoys the selectivity, sensitivity and versatility of molecular synthesis as well as the benefits of robustness and proven technology of today's optoelectronics (to this end see, e.g., “Physics, Chemistry, and Technology of Solid State Gas Sensor Devices”, by A. Mandelis and C. Christofides, Wiley, New York, 1993).
Generally, the design of sensing devices is aimed at achieving sensitivity, selectivity, robustness and versatility. However, combining these qualities in the process of designing an electronic sensor was proven very difficult and many of today's sensors are optically in nature, requiring a detector system to couple to electronic circuits. The use of molecules (e.g., organic molecules) as sensing elements in electronic detectors has the benefit of versatility and selectivity, but is not associated with robustness, especially when organic molecules are employed, because in many cases electrons are required to flow through the organic medium, thus causing the destruction of the sensing element. In other cases, e.g., when no organic medium is employed, or when no electron flow through the organic medium takes place, sensitivity becomes the limiting parameter. Generally, sensitivity of sensors is proportional to the contact area of the sensitive surface, because the larger the area the higher the probability that a molecule or photon can be detected by the sensing surface. Thus, in general, sensitivity is assumed to scale with surface area.
In Molecular Controlled Semiconductor Resistors (MOCSER), the molecules are adsorbed directly on the surface of a semiconductor device [U.S. Pat. No. 6,433,356, Gartsman, K. et al., Chem. Phys. Lett., 283:301-306, 1998]. This is in contrast to most other devices, where the chemicals are adsorbed on the gate metal or insulator layer of a metal oxide FET (MOSFET), or on the surface metal of a Schottky diode. These devices have limiting sensitivity because of the insulating film of the MOSFET and/or the metal.
The combination between molecules and semiconductor devices is also relevant for biological systems, as it allows use of molecules irrespective of their ability to form good monolayers, their electrical conductivity or their stability against electron transport through them [Ashkenasy et al., Acc. Chem. Res., 35, 121-128, 2002]. The ability to affect electronic properties of a semiconductor surface by adsorption of layers of (organic) molecules has been demonstrated and used to achieve selectivity.
Apart from the MOCSER several molecular semiconductor sensors are known in the art. Chemically modified FET (CHEMFET) sensors are based on changes in the current passing through the device due to adsorption of molecules on the gate. Un-gated (open gate FET; OGFET) sensors or surface accessible FET (SAFET) sensors include molecules, which are adsorbed on the surface normally covered by the gate metal between the source and the drain. These types of sensors, however, suffer from over-sensitivity to electrical interference due to their open gate structure, leading to high noise levels compared to devices with channels completely covered by a gate oxide and/or metal.
Most prior art FET sensors use MOSFET-like structures because the relatively low barrier height that characterizes silicon devices leads to high leakage currents. An intrinsic problem one faces with MOSFET-like structures is that the oxidation layer on the surface reduces the sensitivity to adsorbed chemicals. This problem can be overcome by using molecular layers for both noise reduction (surface stabilization) and gating the FET, as disclosed in U.S. Pat. No. 6,433,356 and Gartsman et al. supra. Best results were obtained by constructing MOCSERs from special GaAs/(Al,Ga)As structures, which are similar to high electron mobility transistors, but without a gate electrode. Apart from the cost, use of GaAs and related materials leads to problems for in vivo use of such sensors and to ease of incorporation in present day Si-based electronic technologies.
There is thus a widely recognized need for, and it would be highly advantageous to have a molecular controlled electronic sensor, devoid of the above limitations.